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G a can talk to b and b can talk to c but a cant talk

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Unformatted text preview: means the system can be offline for a short period if the leader fails, as everyone will need to wait for the lease to expire before proceeding. 5) How does a leader failure affect clients? Client request can be in various states: Chosen and learned and returned back to the client – done Chosen and learned and not returned to the client – client asks new leader for status Chosen and but not learned – client asks new leader for status, and new leader replies after the result has been learned Accepted by some acceptors, but possibly chosen or possibly not chosen – client asks new leader, and new leader attempts to resolve request, possibly inserting a no op in previous items and asking client to retry If client request can be proven to have not been chosen (e.g., no live nodes have accepted the proposal to slot that request at any instance, and a majority of nodes are live), then new leader can reply to client: just try again. Client request gets assigned a new tentative instance #, they vote, and agree on its position. That’s consistent with linearizability, since result hadn’t been conveyed back to the client. But! Needs to do this in a way that guarantees no sequence of failures will cause client request to be done twice! E.g., need to elect a no- op at 57 to guarantee that if old leader recovers, it won’t try to vote in A at 57. That makes it safe to put A at some new instance #. 6) changing the set of voters a node crashes permanently and needs to be replaced a node finds that its file system has been corrupted a new...
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